The invention relates to a method of casting metallic parts and to a device for use in the casting method. More specifically, the present invention relates to a method and a device to homogenize, to improve the microstructure and to control the temperature of casting material in a casting process.
High-pressure die casting (HPDC) is a process in which liquid alloy is injected from a prep device, known as a “shot chamber”, into part cavities in a mold at high speed and high pressure. Because of its short cycle time, near net shape and capability for making multiple parts in one shot, HPDC is one of the most economic processes to produce high-volume alloy products. However, HPDC products often contain defects, e.g. porosity, oxide inclusion and cold shot, which are not acceptable for applications that require high strength or leak tightness.
Squeeze casting is an improvement of HPDC where the mold is maintained at higher temperature and the molten metal is injected upward against gravity at a slower speed into the cavity to maintain a laminar flow that progressively fills the cavity. Although squeeze casting is capable of producing parts with improved quality, the cost of squeeze-casting products is very high due to much longer cycle time and substantially shorter die life.
Another casting process, thixocasting is a semi-solid process in which the alloy is pre-cast with electromagnetic stirring to obtain a non-dendritic alloy microstructure and then partially re-melted to a semi-solid state before being injected into the mold cavity. As semi-solid metal has high viscosity, small shrinkage and good fluidity, cast products can be produced with improved near net-shape and less porosity. However, as the cost of the special feedstock and the re-melting process is high, thixocasting is not cost competitive.
Rheocasting is another type of semi-solid casting process in which semi-solid metal with non-dendritic microstructure produced from liquid metal is charged directly into a HPDC press for casting. Conceptually, rheocasting could be a cost-competitive process with good product quality.
However, the latent heat of liquid metal is typically very high. Consequently, the requirement to cool liquid metal quickly into semi-solid status without causing a large temperature difference is rather challenging. As the rheology of semi-solid metal is very sensitive to temperature, the resulting temperature differences in the semi-solid metal could cause unacceptable defects, e.g. cold shot, mend line and porosity. Furthermore, a rheocasting system is very complex and requires possible down time to contain and to transfer the semi-solid metal.
Thixomolding is another semi-solid process in which solid alloy pellets are sheared, melted and transported forward along a heated barrel by a rotating screw. When sufficient material accumulates in the shot chamber, the screw moves forward to inject the molten alloy into a steel mold. Because the screw is exposed to molten alloy at high temperature, thixomolding is not compatible with corrosive alloys, e.g. aluminum. In addition, the quality of thixomolding products are not appreciably better than HPDC, as the injection force for a thixomolding machine is typically lower than that for a HPDC machine with the same clamping force.
Further, for metal-matrix composites, where harder particles, e.g. silicon carbides, are added into lightweight alloys to improve mechanical properties, existing HPDC and squeeze casting processes are unacceptable as the solid particles may have segregated from the alloy matrix due to density difference in the accommodating chamber of a die-casting machine before the composite is injected into the mold cavity.
The use of electromagnetic fields in metal processing, especially in continuous casting, has been explored for many decades.
For example, U.S. Pat. Nos. 2,861,302, 2,877,525 and 3,693,697 taught methods to improve a metal's microstructure in continuous casting by applying stationary, rotating or linearly shifting electromagnetic fields, respectively, to stir liquid metal. In U.S. Pat. No. 4,321,958, a rotating electromagnetic field and a linear electromagnetic field were combined to create a spiral stirring pattern in metal. In U.S. Pat. No. 4,645,534, the electromagnetic field was applied to maintain a sharp interface between two metals cast continuously in an ingot.
U.S. Pat. No. 3,467,166 discloses how to replace a physical casting mold with shaped conducting coils by forming a gap between the coils and the cast ingot with an electromagnetic field. In U.S. Pat. No. 4,678,024, an electromagnetic field is applied to prevent liquid metal from leaking through the gap between two rollers. An electromagnetic field was applied to pump liquid metal in U.S. Pat. No. 4,776,767. In U.S. Pat. No. 4,986,340, an electromagnetic field is applied as a brake to slow down the metal flow for more uniform speed in continuous casting.
U.S. Pat. No. 4,229,210 teaches a method of producing a semi-solid slurry in a crucible through agitation induced by generating an alternating electromagnetic field with a solenoid coil. U.S. Pat. No. 5,579,825 suggests a similar method to produce semi-solid metal in a HPDC machine with a shot chamber that does not allow electric current to circulate. As Winter et. al. pointed out in U.S. Pat. No. 4,434,837, a high-frequency electromagnetic field can only penetrate a small depth into a metal's surface. Hence, induction agitation can only modify the microstructure of alloy near the shot chamber walls. The microstructure of-the alloy beyond the penetration depth remains dendritic, especially for crucible or shot chamber with larger diameter. Furthermore, the high heating energy generated by the eddy current only makes it more difficult to cool the metal from a liquid into a semi-solid state. U.S. Pat. No. 4,434,837 teaches a process to produce semi-solid metal by stirring liquid metal with a rotating electromagnetic field in a crucible under controlled cooling. A similar method was suggested in WO 01/91945 to produce semi-solid metal billets and to transfer the material into the shot chamber of a HPDC machine to produce parts.
As Winter et al. points out in U.S. Pat. No. 4,434,837, the stirring efficiency of the shifting electromagnetic field decreases rapidly as the metal temperature decreases and the corresponding viscosity of the semi-solid metal increases.
In fact, Winter et al. U.S. Pat. No. 4,434,837 reported that the semi-solid metal in the periphery stopped shifting first and that the non-shifting portion gradually propagated toward the center of the casting.
When this method is applied to a rheocasting process, as described in WO 01/91945, it is likely that the colder dendritic metal on the periphery may be injected, along with other metal, into the product cavities and cause defects.
U.S. Pat. No. 6,135,196 is a slurry process in which semi-solid metal is prepared in a first chamber and drawn by a vacuum into a second chamber where a ram injects the slurry into the mold cavity. The disclosed machine is rather complicated and a vacuum may not provide sufficient force to draw a semi-solid metal with high solid fraction from the first chamber into the second.
In U.S. Pat. No. 6,165,411, the slurry preparation was divided into three stages: (1) nucleation of equiaxed crystals by pouring liquid metal into a cup; (2) crystal growth under air cooling and induction heating; and (3) re-melting by induction heating.